text: Diels–Alder reactions occur between a conjugated diene and  an alkene (commonly known as the dienophile). This cycloaddition process allows for the stereoselective formation of cyclohexene rings capable of possessing as many as four contiguous stereogenic centers.
  Diels–Alder reactions can lead to the formation of a variety of structural isomers and stereoisomers. Molecular orbital theory considers that the endo transition state, instead of the exo transition state, is favored (endo addition rule). Also, augmented secondary orbital interactions have been postulated as the source of enhanced endo diastereoselection.
  Usually, CLAs are employed to activate the dienophile.  A typical CLA catalyst is derived from a Mg center made chiral by attachment of a binol- phosphate ester. CLAs have been applied to a number of intramolecular Diels–Alder reactions.
  A complex derived from diethylaluminium chloride and a “vaulted” biaryl ligand below catalyzes the enantioselective Diels–Alder reaction between cyclopentadiene and methacrolein. The chiral ligand is recovered quantitatively by silica gel chromatography.
  The chiral (acyloxy) borane (CAB) complex is effective in catalyzing a number of aldehyde Diels–Alder reactions. NMR spectroscopic experiments have indicated close proximity of the aldehyde and the aryl ring. Pi stacking between the aryl group and aldehyde has been suggested as an organizational feature that imparts high enantioselectivity to the cycloaddition.
  Bronsted acid-assisted chiral Lewis acid (BLA) catalyzes a number of diene-aldehyde cycloaddition reactions.